Most coronavirus rules will remain in place in England for another four weeks after the planned 21 June unlocking, government sources have told the BBC.
Senior ministers have signed off on the decision to delay the lifting of all legal restrictions on social contact.
That could mean capacity limits for sports, pubs and cinemas will remain, and nightclubs would stay closed.
Prime Minister Boris Johnson is due to confirm the delay at a news conference later – at 18:00 BST.
The extension will be put to a Commons vote this month and could trigger a sizeable Conservative backbench rebellion.
England was due to move to stage four of the government’s roadmap out of lockdown on 21 June, when venues and events would be allowed to operate without capacity limits and the cap on guests at weddings would be lifted.
It would also have meant an end to current rules which limit gatherings at home to six people or two households.
But many scientists have called for the reopening to be delayed to enable more people to be vaccinated and receive second doses, amid rising cases of the Delta variant, which was first identified in India.
A delay would also allow more work to be done on whether vaccines are breaking, or simply weakening, the link between infections and hospitalisations.
Health Minister Edward Argar told BBC Breakfast that he could not confirm the delay before the PM’s announcement, but that there was a “concerning increase” in cases of the Delta variant and numbers in hospital were “beginning to creep up”.
Most severe cases were among unvaccinated people or those who had only one dose, he said, adding that at current rates nearly 10 million second doses could be administered over four weeks to increase protection.
Mr Argar said the prime minister would address issues of economic support in any announcement and that the PM was “very sensitive” to the situation of couples who had already had to postpone their weddings.
All areas of Scotland are due to move to Level Zero Covid restrictions on 28 June – meaning bigger groups can gather in cafés, pubs and restaurants, although they will still have to observe social distancing.
Limits on indoor gatherings in Northern Ireland are scheduled to be relaxed on 21 June and the current rules in Wales will be reviewed on 25 June.